The present invention relates to an arrangement of an antenna in a metallic environment.
The invention more particularly relates to the arrangement of an antenna in the general form of a loop, of the type comprising at least one turn; the said antenna extending directly in the vicinity of and substantially parallel to at least one metallic element.
It is known from the state of the art that the presence of metal in the immediate vicinity of an antenna in the form of a loop generates disturbances with regard to the transmission and/or reception of the electromagnetic waves. This is because the function of such an antenna is generally to transmit or pick up a field with a dominant magnetic component. If it is positioned in the direct vicinity of a metallic element, the latter will naturally have a tendency to behave like a turn in short circuit. The performance of the antenna will consequently be degraded, or even completely destroyed.
The use of antennae in the form of a loop is becoming widespread and finds many applications since they are found for example in the body of microprocessor cards of the contactless type, in the support element for so-called xe2x80x9cintelligentxe2x80x9d electronic labels, but particularly in many readers and other radiofrequency communication devices.
For many applications, such as for example access control or vending machines, it is important for the antennae and the associated electronics to be able to protected against vandalism and consequently to be integrated into a metallic environment.
In order to avoid the problems mentioned above, that is to say those resulting from the influence of the metallic environment on the functioning of the antenna, it has already been proposed to interpose a layer of xe2x80x9cabsorbentxe2x80x9d material between the antenna and the supporting metallic element. However, it is generally a case of an expensive material such as ferrite, which substantially limits the advantage of this solution.
More generally, the antenna is intentionally kept distant from the metallic environment, the relative positions of these two elements being determined so as to obtain a satisfactory functioning of the antenna. This type of arrangement does however have the drawback of considerably increasing the total bulk of the whole.
Thus the technical problem to be resolved by the object of the present invention is to propose an arrangement of an antenna in the general form of a loop, directly in the vicinity of at least one metallic element, an arrangement which will make it possible to avoid the problems of the state of the art by offering minimum bulk, whilst still having a limited production cost.
The solution to the technical problem posed consists, according to the present invention, of each metallic element having on the one hand an aperture provided substantially opposite the surface delimited by the antenna and on the other hand a slot forming an air gap, formed through the thickness of the metallic element, between the internal edge delimiting the aperture and the external edge of the said metallic element.
The combination of the aperture and the air gap formed on each metallic element makes it possible to avoid any phenomenon of shielding of the antenna by the said metallic element. The invention as thus defined has the advantage of guaranteeing perfect functioning of the antenna in a metallic environment, even in the absence of any interposed absorbent material or any significant separation between the said antenna and the metallic element in question. Thus the antenna therefore offers identical or equivalent performances to those which it would have in a non-metallic environment, whilst benefiting from the advantages peculiar to the use of metal, notably in terms of security and more particularly resistance to vandalism.
In a particularly advantageous fashion, the antenna lies substantially parallel to the metallic element of which it is directly in the vicinity, in order to limit the bulk of the whole to the maximum possible extent. The antenna and the contiguous metallic element are thus positioned side by side with a small intervening space, which may even be zero.
According to one particularity of the invention, the surface delimited by the internal edge of the aperture is substantially equal to the surface of the antenna. The characteristics of the aperture with regard to shape and dimensions have in fact a direct impact on the performance of the antenna, since the passage of the electromagnetic waves takes place precisely through the said aperture, independently of the essential presence of the air gap.
According to another particularity of the invention, the internal edge of the aperture is shaped so as to be substantially parallel to the contour of the antenna. In other words, the shape of the aperture corresponds substantially to the projection of the contour of the antenna onto the metallic element. An optimised functioning is in fact obtained when the respective surfaces of the aperture and of the antenna are equivalent.
In a particularly advantageous fashion, the internal edge of the aperture coincides with the mid-axis of the loop forming the antenna. The shape of the aperture corresponds here to the projection of the mid-axis of the antenna onto the metallic element. It should be noted that the mid-axis designates the median line passing longitudinally through the loop, and this whatever the number of turns making up the antenna.
This conformation constitutes an optimum solution in that, on each side of the mid-axis, that is to say in the case where the shape of the aperture is larger or smaller than this limit, the performance of the antenna is significantly reduced.
According to one variant of the invention, the antenna extends between two metallic elements, with respect to each of which the said antenna is positioned directly in proximity. Naturally, each metallic element is in accordance with the invention, that is to say it has an aperture and an air gap as defined previously.
This configuration, in which two metallic elements are positioned on each side of the antenna, corresponds for example to the integration of an antenna inside any metallic part, such as a watch case for example.
According to another variant of the invention, the antenna lies outside an assembly including two metallic elements between which a space is formed, able to receive a removable device also provided with an antenna in the form of a loop; the external antenna being positioned directly in the vicinity of the external face of one of the metallic elements. This configuration can concern, for example, an insertion reader, or a contactless card or electronic labels.
According to another characteristic of the invention, the different metallic elements being fixed together, the respective apertures and slots are associated so as to constitute a single air gap. The important thing is that the air gap peculiar to each metallic element is not short-circuited by any portion of the adjacent metallic element. This results in positioning the slots substantially facing each other, and this whatever their respective dimensions. Thus it can therefore be considered that the joined-together metallic elements have only one air gap.
Advantageously, the metallic elements intended to be positioned close to the antenna may be symmetrical. This means that their identical shapes can be positioned easily opposite each other, symmetrically with respect to the antenna, so as to be able to have very simply a single air gap. The apertures of the different metallic elements, just like the slots also, are then positioned precisely opposite each other.
According to another particularity of the invention, the different metallic elements belong to the same piece. This configuration is of course equivalent to that for which the different parts, coming to be positioned close to the antenna, are joined together.